1. Field of the Invention
The invention relates to a direct current motor controlling method, and particularly to a method for controlling output performance of a direct circuit motor.
2. The Related Art
Nowadays direct current (DC) motors are utilized in many apparatuses such as a computer fan, a car fan, an electronic bike and a hair dryer etc. The requirement of the DC motors is altered by different fields. However, output performance of the DC motor is most important.
A conventional controlling device for controlling output performance of the DC motors is disclosed in FIG. 7. The controlling device 900 includes a microprocessor unit (MPU) 901 and a pulse width modulation (PWM) unit 903 and a DC motor 905. The MPU 901 receives a duty signal having a cycle time value and a work time value of the motor. The MPU 901 computes the cycle time value and the work time value and then produces a duty value through the following function:dutyvalue=worktime÷cycletime×100%
Referring to FIG. 8, in this case, the duty signal of the motor is a square wave signal. The MPU 901 receives the duty signal having the work time value of the motor and the cycle time value of the motor and then the duty value based on the fetched work time value and cycle time value will be further computed by the above-indicated function. In this case, the duty value is 10%.
Please refer to FIG. 9. After the MPU 901 generating the computed duty value as 10%, the MPU 901 compares the computed duty value to all duty values listed in a data table in order to find out a target value corresponding to the computed duty value. Based upon the table shown in FIG. 9, it takes 10 steps for the MPU 901 to find out the target value as 31 when the computed duty value is 10 (means 10%). However, when the computed duty value is 90, the MPU 901 must execute 90 processing times for finding out the target value which is 133. Once the target value is retrieved, the MPU 901 sends the target value to the PWM unit 903. The PWM unit 903 receives the target value and then transmits a variable frequency signal to a DC motor 905 according to the target value for controlling output performance of the DC motor 905.
However, the controlling device 900 sustained defects, such as: 1) the process of computing the cycle time value and the work time value and then producing the duty value through the above-indicated function increases executing load and executing time of the MPU 901 because of the above-indicated function including multiplication and division; 2) because the data table saved the duty value and the target value stored in the memory of the MPU 901, the used memory space of the MPU 901 is increased, and 3) the process of comparing the computed duty value to all the duty values listed in the data table step by step for getting the target value increases executing time of the MPU 901.